Pasteurization has been used in the wine and brewing industry for many years having been developed around 1865 by Louis Pasteur. Pasteurization is the reduction of microorganisms by heating to a limited temperature and holding at that temperature such that there will be minimum effect on physical stability and flavor and a maximum extension of biological stability, thereby increasing shelf life.
Although the present application describes the invention with reference to the pasteurization of beer, the invention also is applicable to all products which are pasteurized in a package, including soda and other soft drinks, fruit juices and drinks, and milk. The specific P.U. inputs and temperatures and time will vary according to the product being treated.
The various organisms in beer are not pathological and dangerous to humans, but affect the taste and appearance of the beer if allowed to grow. Draft beer generally is not pasteurized because it is kept refrigerated and is usually consumed in a short time. However, high quality can and bottle beer traditionally is pasteurized for long shelf life, and conventional type pasteurizers are shown in Herold U.S. Pat. No. 2,282,187 issued May 5, 1942 and Wehmiller U.S. Pat. No. 2,658,608 dated Nov. 10, 1953.
In this type pasteurizer, water is sprayed onto the tops of the closely packed packages which are moved through a tunnel which is divided into a series of preheat zones, a series of heating zones, a holding zone, and a series of cooling zones. The temperature of the beer in the containers is progressively raised to the desired level before being passed through the cooling zones where it reaches the desired beer out temperature (BOT). The water running off the packages is collected in reservoirs, heated or cooled, and recycled to the sprays or spray pans. These types of tunnel pasteurizers are available in single and double deck configurations. The spray nozzles on the lower decks traditionally are difficult to maintain free from blockage, so the single deck conveyor has been the generally accepted norm in the brewing industry in the United States.
If pasteurization causes the temperature of the beer to reach too high a level, certain tastes called "pasteurization tastes" can occur in the beer. These are undesirable and have been defined as "bready", "biscuity", "burnt-type tastes", "papery" or "cardboardy". Also, insufficient pasteurization can result in turbidity of the beer or sedimentation.
The organisms which cause the most difficulty in the brewing industry today are lactobacillus, pediococcus and wild yeast. These are not pathogenic organisms but can cause turbidity and poor taste in beer. In order to insure complete pasteurization of beer, the temperature of the beer at the so-called "cold spot" which is 1/4" from the bottom of the center of the can or bottle must reach at least 140.degree. F. for a period of time to produce a specified number of pasteurization units (P.U.). It also is desired to pasteurize at the lowest possible peak cold spot temperature (above 140.degree. F.) to avoid overheating the rest of the package contents, because the temperature of the package gets higher as one goes from the cold spot to the top of the package.
The P.U. is a measure of accumulated lethality. One P.U. for beer is one minute at 140.degree. F. Lethality (P.U. per minute) is a rate term which is exponential with temperature. Lethality begins to become significant only when the beer temperature is 132.degree. F. to 135.degree. F. and is most significant at 139.degree. F. and above, although P.U. accumulation begins at 120.degree. F.
Over the years different minimum P.U. have been stated as the requirement for pasteurizing beer, but it is clear that at least 5.6 P.U. are required and if numerous organisms are present, a standard of 8 P.U. has been set forth. Some breweries require a minimum of 10 P.U. as their standard. It has been determined that Lactobacillus Brevis is the most heat resistant bacteria normally occurring in beer. It is this organism that brewery pasteurization of beer requires 140.degree. F. as the minimum standard temperature for pasteurization.
Conventionally, some pasteurizers have used spray densities as low as 3.4 gal./min./ft..sup.2 of tunnel surface area and have used spray heads aligned in a straight line running the length of the tunnel. Recent precise measurements of the spray pattern developed in such pasteurizers have shown a wide variance of water applied to the different cans or bottles across the width of the tunnel. Thus, the product will receive varying and insufficient water spray coverage, resulting in non-uniform heating (or cooling) of the product. This in turn results in non-uniform product pasteurization. It is desired to maintain a uniform P.U. input to the packages, i.e., a standard deviation of P.U. input of two or less.
As used in the brewing industry in reference to P.U. input, the term, standard deviation, means that the P.U. input to all packages varies by only the stated amount. Thus a standard deviation of P.U. input of two or less indicates that the P.U. input to any package in the line varies by only .+-.2 P.U. at the most. This is not the same meaning the term standard deviation has when used in the fields of statistics, probability, etc.
The theory of transfer of heat from the water spray to the contents of the container is that the film resistance to transfer is greatest at the inside wall of the container and mixing in the container occurs through thermal conduction and convection currents generated as the contents are heated and circulated. The circulatory motion across the heat transfer surfaces reduces the internal surface film resistance and tends to bring the container contents to an equilibrium temperature. The so-called "coldspot" in the container is stated to be 1/4" above the center of the bottom wall and is the last point to come to equilibrium temperature through these conduction and convection currents. It is necessary to raise the temperature of the cold spot to above 140.degree. F., but is desirable to hold it as close to 140.degree. F. as possible to minimize taste changes caused by excess heating of the remainder of the container.
Accordingly, it is a principle object of the present invention to provide pasteurization apparatus in which there is very little deviation in P.U. applied to each package. It is a further object to maximize heat transfer efficiency by applying a uniform amount of water to each package and to provide increased intensity of water flow over the product. It is still another object to provide the lowest practical beer out temperature for the product leaving the pasteurizer.
It is still another object of the present invention to provide an improved double deck pasteurizer in which only a single set of upper deck spray heads is required with the overflow from the upper deck dripping onto the containers on the lower deck in a volume and at a temperature such that the P.U. input between the two decks is substantially the same.
Still a further object is to provide a tunnel pasteurizer in which the pattern of spray heads is staggered across the surface area of the tunnel such that the flow of heating and cooling water is uniform onto the containers on the deck. It is still another object to provide a pasteurizer tunnel such that the flow through the spray heads is at least 7 gal./min./ft..sup.2 of tunnel surface area. A further object is to provide a pasteurizer apparatus such that the first cooling zone is regenerative with the first heating zone, i.e., the zone where spray temperature is first at 140.degree. F., to more rapidly heat the product toward 140.degree. F. and allow a longer holding period at a lower product cold spot temperature as close to, but greater than 140.degree. F. as possible, thereby reducing the maximum hot spot temperature while maintaining the P.U. input to the contents of the container.
These and other objects and advantages will become apparent hereinafter.